Variable aperture, variable flux density, aerospace solar collector

ABSTRACT

A trough-type reflecting solar concentrator and receiver system for aerospace use is disclosed. The reflecting surface is a thin flexible sheet attached to and disposed between curved ribs. The amount of the sheet reflecting sunlight can be changed by winding the unnecessary quantity of sheet onto a roll. The focal length of the curved ribs and therefore the sheet can be changed by flexurally deforming the curved ribs by applying end loads, causing a change in the flux density at the receiver. A similar result is achieved by changing the distance between the reflecting surface and the receiver. The receiver is a photovoltaic array, thermal absorber, or a combination of both. Means for proportioning the amount of energy incident on each type of receiver is disclosed. A spectral splitting thermal absorber with scattering capability is disclosed as a pre-filter for the photovoltaic array.

This is a continuation of application Ser. No. 06/800,219 filed11/21/85, now U.S. Pat. No. 4,719,903.

BACKGROUND

The program to explore outer space has increasing requirements forelectrical and thermal power. The conventional photovoltaic flat platearray may not be the optimal choice to supply the significantly largerquantities of power required by the next generation of space systems.One group of candidate replacement systems uses a curved reflector tofocus the sun's energy onto a thermal absorber which transfers theenergy to a working fluid that drives a heat engine/electric generator.This system promises higher overall efficiency with the disadvantage ofincreased complexity. The higher efficiency allows a smaller reflectoraperture area for a given power output. This is a major advantage sinceit minimizes launch mass and low orbit aerodynamic drag. Low orbitaerodynamic drag is a major factor since it causes the orbit to decay,requiring a fuel-consuming rocket firing to re-achieve the originalorbit. Therefore, the lower the required aperture, the lower theaerodynamic drag, the lower the mass of re-boost fuel that mustlaunched.

Lightweight reflectors may be made by using a thin film as thereflective material. Of particular interest are those disclosures thatform the film into a parabolic trough reflector. Simpson in U.S. Pat.No. 4,173,397 describes a thin film reflector sheet (22) stretched overwires (20) or rods (70) in a terrestrial environment protected from windand rain. A major disadvantage of this invention is the lack of a smoothcontour. The reflective areas between the wires or rods are planar (seeFIG. 3); therefore, the reflector is an array of flat facets. Thisseverely limits the optical concentration potential and therefore theoverall effeciency potential of this device. Cohen in U.S. Pat. No.4,184,482 describes a trough reflector for use under terrestrial roofrafters. The film (30) is stretched between two frames (32) which areheld apart by a biasing spring (64) at the tubular member (24). Thetrough reflector assembly rotates about the tubular member (24) to trackthe sun. Bronstein in U.S. Pat. No. 4,293,192 describes a stretched filmterrestrial trough reflector made of a reflector sheet (46) attached toforms (22) and (24) which are moved apart on a slideway (26) by a nut(38). Eaton in U.S. Pat. No. 4,493,313 describes a terrestrial troughcollector using a stretched film (16) attached to parabolic end formers(18) which are forced apart by a spreader bar (12). All of the foregoingare directed to terrestrial use and are not adaptable to transportationinto space or use in space. All of the foregoing have a rigid connectionbetween the film or foil and the end formers such that the aperture areacannot be changed. Also, the end formers are rigid (rather thanflexible) with respect to their curved (parabolic) shape, therebycreating a reflecting surface with a fixed focal length.

SUMMARY OF THE INVENTION

One object of this invention is to provide a light-weight, parabolictrough solar collector that is readily transportable into space andreadily erectable or deployable in space, particularly in low earthorbit. Another object of this invention is to provide a parabolic troughsolar collector that has a variable aperture such that its collectingpower capability and its aerodynamic drag can be changed. Another objectof this invention is to provide a parabolic trough solar collector witha combination of electrical and thermal energy converters and means tovary the percentage of energy striking them. Another object is toprovide a parabolic trough solar collector with a variable focal lengthreflector to allow the flux density and distribution at the focus to bevaried.

The present invention uses a curved reflector of the trough type tofocus the sun's energy onto an elongated receiver assembly. The curvedreflector is composed of a stretched reflective film attached totransverse ribs that are in a parabolic shape. The receive assembly iscomposed of a photovoltaic cell array with a spectrally selectivefilter. The spectrally selective filter is a thermal absorber using acirculating fluid. It absorbs those portions of the sun's spectrum towhich the photovoltaic cell is not responsive and transmits thoseportions of the spectrum to which the cell is responsive.

Another embodiment uses flexible transverse ribs that can be made tohave an adjustable focal length. The focal length determines the energydistribution entering the receiver assembly. In this embodiment, thereceiver assembly contains an array of absorbers that convert theconcentrated flux into electrical and/or thermal power. The amount ofenergy incident on each absorber can be varied by changing the focallength of the reflector and/or the alignment of the reflector axis withthe sun.

Another embodiment allows the amount of stretched reflective filmmaterial to be varied. This allows the collection aperture to betailored to the electrical and thermal power loads as they vary overtime. Therefore, the aperture area and the associated aerodynamic dragare minimized for a given mission profile. This minimises the fuelrequirements to re-acquire the original orbit after orbital decay due toaerodynamic drag.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an end view of the solar collector, looking parallel to thelongitudinal axis.

FIG. 2 shows a top view of the solar collector as it would be seen ifviewed from the sun. The longitudinal axis is vertical.

FIG. 3 shows the receiver assembly in cross-section with the in-line(serial) thermal and electric converter embodiment.

FIG. 4 shows the receiver assembly in cross-section with the abreast(parallel) thermal and electric converter embodiment.

FIG. 5 shows a cross-sectional view of a rib member, the film, and thesliding attachment between them.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show the preferred embodiment of the solar collector intwo views. The reflecting material is a thin film (1) of silvered oraluminized Mylar or Kapton (both are trademarks of the DuPont Co.)stretched, in the direction of the longitudinal axis, between andattached to elastically deformed curved rib members (3), which have axestransverse to the longitudinal axis. Alternatively, the reflectingmaterial can be a thin metallic or plastic sheet. The curved rib members(3) are attached at their ends to the shafts (24) of torque producingmotors (4). The curved rib members are initially straight and willreturn to the straight condition when they are not stressed; therefore,they are stressed only within the elastic range of their materialproperties. The frames or bodies of the motors (4a) at one set of endsof the curved rib members (3) are rigidly fixed to transverse framemembers (6). The frames of motors (4b) at the other set of ends of thecurved rib members (3) are mounted to blocks (not shown) which arecaptured in slots (18) in the transverse frame members (6) such thatthey may move toward the center of the collector as they exert torque onthe curved rib members (3) and curved rib members (3) deform into acurved shape. The theoretical development of this process is morecompletely described in my U.S. Pat. No. 4,119,365, which isincorporated by reference. The transverse frame members (6) are spacedapart by elongated longitudinal frame members (5), which are attached atthe ends of the transverse frame members (6), and which are parallel tothe longitudinal axis. Receiver struts (7) are attached to each of thetransverse frame members (6) at their midpoints, and support thereceiver assembly (25) and the receiver cover (8) at their oppositeends. Alternatively, the receiver assembly (25) could be supported bymultiple struts, to form a truss for increased structural stiffness, ifrequired by the orbital dynamics. Light from the sun approaches thesolar collector with virtually parallel rays (20) which are reflected bythe film (1) to focus converging rays (21) at the receiver. Only oneside of this optical process is shown for clarity in FIG. 1. The film(1) is made from a continuous roll of film. To each transverse edge ofthe film(1) is attached a sliding member (15) which is a continuouselongated member with a hollow longitudinal core (19) as shown in FIG.5. The film (1) is wrapped into a small loop (22) and placed in thehollow core (19). A thermosetting filler/adhesive (16) such as an epoxycompound or the like can be injected from one end to fill the hollowcore (19) which is now lined with the film loop (22), thus firmlyattaching the film (1) edge to the sliding member (15). The slidingmember (15) is captured in a box member (14) which has a slot for thefilm (1). The box member (14) is attached to each curved rib member (3),such that it deforms in the same manner as the curved rib member (3).Supported by lugs (17) attached to the transverse frame members (6) isthe film roll shaft (12) which in turn supports the film roll (2).Coupled to the film roll shaft (12) is the armature of the film rollmotor (13). The stator or frame of the film roll motor (13) is attachedto the lug (17).

One edge of the film (1) parallel to the longitudinal axis and theadjacent ends of the sliding members (15) attached to the transverseedges of the film (1) are attached to the film roll (2). By energizingthe film roll motor (13), the film (1) and the attached sliding members(15) may be wound onto the film roll (2). During this operation, thefilm (1) and the sliding members (15) are retracted in the box members(14). This decreases the effective collecting area or aperture of thereflector; reducing the output power potential to that required by thespacecraft at that point in time; and also reducing the area exposed toaerodynamic drag, thereby reducing the drag losses. The edge (26) of thefilm (1) in the partially retracted position is shown in FIG. 2. FIG. 3shows the preferred embodiment of the receiver of the concentrated solarflux (21), which focuses at a location (23) in front of the receiveraperture. As the flux diverges from the focus location (23), it strikesa photovoltaic cell (10) or the like which converts the flux intoelectrical energy. The flux also strikes and is absorbed by two thermalreceivers (11), in the form of hollow conduits transporting a heattransfer fluid such as water or a high temperature oil, that convert theflux into thermal energy. As shown in FIG. 3, the focus location (23)causes flux to strike the photovoltaic cell (10) and the thermalreceivers (11). If the effective radius of curvature of the reflectorwere lengthened by decreasing the torque applied by the motors (4) onthe curved rib members (3), the focus location (23) will move toward thereceiver aperture. As this happens, decreasing amounts of flux fall onthe thermal receivers (11), and a more concentrated flux falls on thephotovoltaic cell (10). In this manner, the proportion of the incomingenergy to each type of device can be changed to match the prevailingload requirements. Similarly, the entire collector assembly may berotated in spaced to be slightly misaligned to the sun, thus causing thefocus location to move laterally left or right in FIG. 3. This wouldcause a shift in the power, proportioning toward the thermal receivers(11), Also, the focus location (23) can be placed very close to thesurface of the photovoltaic cell (10) and traversed across it bychanging the optical alignment of the collector with respect to the sun.This produces local heating that can anneal the cell, which repairs anyaccummulated radiation damage and acts to rejuvenate the cell. Thethermal receivers (11) and the photovoltaic cell (10) are attached tothe receiver cover (8) by appropriate brackets, not shown. Anothermethod of changing the flux density of the concentrated radiant energyat the photovoltaic cell (10) or any other receiver is to move thereceiver along the optical axis with respect to the reflector while thefocal length is held constant. This can be accomplished by changing thelength of the receiver struts (7) by means of a jack screw or the likeor by repositioning the photovoltaic cell within the receiver assembly(25) by similar means.

FIG. 4 shows another embodiment of the receiver. Concentration flux (21)strikes a selective absorption filter (8) utilizing a circulating fluidin a hollow conduit as the energy transport system. The selectiveabsorption filter (9) absorbs those wavelengths of light to which thephotovoltaic cell is not sensitive and converts the energy to heat whichthe fluid transports to a thermal load. Typically, the photovoltaic cellis not sensitive to ultraviolet or infrared portions of the spectrum.The selective absorption filter (9) is transparent to those wavelengthsof light to which the photovoltaic cell (10) is sensitive, transmittingthem to the photovoltaic cell (10) for conversion to electrical power.The filter (9) and the photovoltaic cell (10) are attached to thereceiver cover (8) by appropriate brackets, not shown. A more completedescription of the selective spectral absorber filter is in my U.S. Pat.No. 4,278,829, which is incorporated by reference. In this embodiment,the fluid is preferred to be the selective absorber itself, therebydirectly absorbing the portion of the spectrum that is non-productivefor the generation of electrical power by the photovoltaic cell (10).

The focus of the concentrated flux (21) is located at the center of theselective absorption filter (9). Although, the selective absorptionfilter conduit is shown as a circular member, other cross-sectionalshapes will provide like functions. Also, the photovoltaic cell may bemanufactured to be transparent to non-productive radiation, such asinfrared. In this case, the concentrated flux would strike thephotovoltaic cell first, and the non-productive radiation would betransmitted to a thermal absorber located behind the cell; i.e. the celland the thermal absorber would be reversed in position from that shownin FIG. 4. Obviously, there are a number of arrangements of absorbingelements, each having advantages for a particular desired result. In theconfiguration shown in FIG. 4, the spectrally selective absorption fluidcirculating in the selective absorption filter (9) should also contain ascattering agent that scatters the transmitted radiant energy as itpasses through the fluid. An example of a scattering agent is a quantityof small glass microspheres being carried by the fluid and having thesame density as the fluid to produce neutral buoyancy. Alternatively,the conduit itself may be etched or "frosted" to produce a "groundglass" effect. The purpose of the scattering agent is to scatter theradiant energy to which the photovoltaic cell is sensitive and therebymake the flux density at the surface of the phototaic cell more uniform.The efficiency of the cell is maximized at the most uniform fluxdensity. This is very important since trough type concentrators producelinear flux variations at the focus of the reflector.

The thermal energy collected from the thermal absorbers (11) may be usedto power to heat engine using an organic fluid such as toluene andoperating on an organic Rankine thermodynamic cycle and driving analternator producing AC electrical power. It will be advantageous to useAC power for some aspects of our future space power requirements whilethe photovoltaic cells provide DC power. Alternatively, if large heatloads must be supplied at reasonable temperatures, it will beadvantageous to transport the thermal energy as sensible heat in a heattransfer fluid and avoid the enormous efficiency losses of convertingfirst to electricity, to be used as the transport medium, then back toheat at the thermal load. The thermal energy may also be transported asa phase change in the fluid, such as water into steam at the heatabsorption point and steam into water at the thermal load.

Although the curved rib members (3) are shown as a constantcross-section elements that are elastically deformed into the parabolicshape, they may be of other configurations such as a structural truss.Small forces may also be applied at the ends and/or other locations onthe curved rib members (3) to `fine-tune` the shape toward a perfectparabola. The curved rib members (3) may also be initially formed intothe parabolic shape, rather than relying on the end moments applied bythe motors (4), to form them into the parabolic shape. If the motors (4)are eliminated, the change in focal length can't be accomplished but allother functions can be performed. A combination of initially curved ribmembers (3) and motors (4) to effect changes in focal length is one ofthe other possible combinations within the scope of this invention.

The variable aperture function produced by the movement of the film (1)with respect to the curved rib members (3) can be produced by relocatingthe film roll (2) to the center of the reflector and unrolling two websthat are interleaved on the film roll (2) in opposite directions.Alternatively, two film rolls could be used at the center or at the theoutboard ends to provide redundancy. Alternatively, the storagemechanism could use an accordion-type pleating of the film (1).

As can be seen from FIGS. 1 and 2, the elements of the solar collectorare slender elongated members, which, when laid side by side, lendthemselves to a compact shipping envelope for transportation into space.Also, the film roll (2) can be used as the transport mechanism for thefilm (1) and the attached sliding members (15). Since there are a smallnumber of joints, the erection in space by astronauts or robots isrelatively simple. After the structural connections are made, the endsof the sliding members (15) are inserted into the ends of the boxmembers (14) and the collector assembly is complete. A number oftechniques are being developed to build structural beams in space and todeploy collapsible/extendable beams. These items could be easilyutilized in my invention.

Using members of less than 18 meters in length to fit into the SpaceShuttle cargo bay would produce an approximately 15 meters by 15 metersaperture with the film fully extended. This would intercept about 315kilowatts of power. At a conversion efficiency of 15%, using galliumarsenide photovoltaic cells, the power output would be 47 kilowatts. Twounits would be required to satisfy the present space station requirementof 75 kilowatts, thereby providing modularity and redundancy.

An alternative embodiment places the receiver assembly with an array ofreflective facets or the like that redirect the concentrated energyreflected from the film (1) to a point focus near the center of thereflective surface where an absorber/converter is located.

While this invention has been described in conjuction with the preferredembodiment thereof, it is obvious that modifications and changes thereinmay be made by those skilled in the art to which it pertains withoutdeparting from the spirit and scope of this invention, and as defined bythe claims appended hereto.

I claim:
 1. A method for annealing photovoltaic cells in space, saidcells being substantially at the focus of a solar energy concentrator,said focus having a flux density and an effective area, comprising thesteps of:increasing said flux density of said focus and decreased saideffective area of said focus on said cells, thereby causing increasedlocalized heating in said effective area on said cells, said localizedheating acting to rejuvenate electrical characteristics of said cells;moving said effective area across said cells, thereby subjectingadditional surface area of said cell to said localized heating.
 2. Amethod as recited in claim 1, wherein said step of increasing said fluxdensity and decreasing said effective area comprises changing the radiusof curvature of said solar energy concentrator.
 3. A method as recitedin claim 1, wherein said step of moving said effective area compriseschanging the optical alignment of said solar energy concentrator withrespect to the sun.
 4. A method as recited in claim 1, wherein said stepof increasing said flux density and decreasing said effective areacomprises changing the distance between the cells and the solar energyconcentrator.
 5. A solar collector for use in space, comprising:a curvedreflector which redirects incoming solar energy to a focus location, aphotovoltaic cell, positioned in proximity to said focus location, saidcell intercepting said energy, said intercepted energy having a fluxdensity and an effective area, means for remotely increasing said fluxdensity and decreasing said effective area, thereby producing localizedheating in said effective area.
 6. A solar collector as recited in claim5, further comprising means for moving said effective area across saidcells, thereby subjecting additional portions of said cells to saidlocalized heating.
 7. A solar collector as recited in claim 5, whereinsaid means for remotely increasing said flux density and decreasing saideffective area comprises means for changing the radius of curvature ofsaid curved reflector.
 8. A solar collector as recited in claim 5,wherein said means for remotely increasing said flux density anddecreasing said effective area comprises means for changing the distancebetween said curved reflector and said photovoltaic cell.
 9. A solarcollector as recited in claim 6, wherein said means for moving saideffective area across said cells comprises means for changing theoptical alignment of said curved reflector with respect to the sun.